Behavioural response of migrating Atlantic salmon smolts when facing hydroelectric barriers and associated hydrodynamic conditions

The factors that influence the behavioural response of the downstream migrating Atlantic
salmon smolts and the choice of a migration route when they are confronted to hydroelectric barriers are important to be understood to enable safe and rapid passage, and to ensure successful migration. We investigated the smolt fine-scale research behaviour during downstream migration at different hydroelectric barriers that offers several migration routes, including safe and potentially lethal routes. To highlight environmental factors that influence the migration route choice, hydrodynamic conditions upstream of hydroelectric barriers were modelling. During smolt tracking, the smolts expressed a wide range of research behaviour. For some smolts, research behaviour turned out to be quite simple and rapid whereas for others, research behaviour was more complex and hesitant. This great inter-individual diversity of the fine-scale research behaviour may indicate the potential influence of the individual's personality, when the smolts were facing an obstacle. Moreover, hydrodynamic modelling highlighted that flow velocity and water depth were used as hydraulic cues in the selection of a migration route, as the smolts preferred higher flow velocities and water depths. The comprehension of the factors influencing the research behaviour at hydroelectric barriers will be useful to design attractive mitigation measures and to guide the smolts efficiently toward safe routes.